It is the elevator deflection that results in the excessive force on the stab screw jack which then may defeat the torque available from the pilot in manual application of trim.
Has anyone actually calculated the torque required to move the stabiliser under maximum aerodynamic load? As the friction forces to be overcome on the threads of the screw-jack will increase exponentially as it moves towards maximum load, adding an electric servo as a solution may need to be done in conjunction with a redesign of the screw-jack mechanism to cope with this additional force.